When a load having a light weight, such as a pot or pan made of non-magnetic metallic material having a small resistance, such as aluminum is heated inductively by a high-frequency magnetic field for heating and cooking an object contained in the load, the load receives an ascending force generated by eddy currents induced by the magnetic field over a heating coil. This force may lift up the load or displace the load laterally during the cooking.
Japanese Patent Laid-Open Publication No. 2001-332375 discloses a conventional induction heater. In the conventional heater, while an output for heating gradually increases from a small level at the starting of the heating to a predetermined level, a change of a source current is monitored to find the lifting and displacement of a load. If the displacement of the load is found, the conventional induction heater performs a control, such as stopping its heating action or decreasing its input power.
FIG. 4 is a schematic view of the conventional induction heater. An inverter 101 drives a switching element included therein to allow a heating coil 102 to generate a high-frequency magnetic field of 50 to 100 kHz as to inductively heat a load 103 made of aluminum. An heating output is changed by controlling a frequency of the driving the switching element.
FIGS. 5A and 5B illustrate a change with time of a power consumed for the inductive heating of the load 103 with the heating coil 102 after the starting of the heating (an input power to the heating coil 102) and a change with time of the source current input to the inverter 101, respectively. According to an increase of the input power to the heating coil 102, i.e., a heating output of the inverter 101, the source current increases. According to the increase of the source current, the ascending force generated by the magnetic field from the heating coil 102 increases, accordingly lifting up the load or moving the load laterally at time P0. Thus, the load departs from the heating coil 102, and the power input to the heating coil 102 accordingly decreases after the time P0. Thus, the gradient of the increase with time of the power input to the heating coil 102 or the source current becomes smaller than that before the time P0.
The value of the source current (peat value or effective value) is measured by a detecting circuit 104. Upon the detecting circuit 104 detecting a change with time of the source current, the inverter 101 stops the heating of the load or reduces the input power, thereby preventing the lifting or displacement of the load.
The conventional induction heater can detect the lifting or displacement of the load at the start of the heating. In other words, the load is not displaced at the start of the heating, thus being heated. The weight of the load may decrease after a long period of time after the start, for example, after water in the load is evaporated or food contained in the load. In this case, the conventional induction heater may fail to detect the change of the weight and continue to heat the load, thereby lifting the load or displacing the load.